MAY 15, 1936
ANALYTICAL EDITION
The action of the indicator is affected by carbon dioxide, but not by a strong oxidizing acid like nitric. The indicator changes from a very light straw color under acid conditions to lemon yellow and finally red as alkali is added, the first change being sharp and the second gradual. The pH range for the change from straw color to red was found Lo be 3.92 to 10.57 and for the change from straw color to lemon yellow 3.92 to 6.34 a t 25' C.
211
The pKa for the first hydrogen was found to be 6.56 and for the second hydrogen 11.33 a t 25' C.
Literature Cited (I) Benedikt, R., Bel., 11, 362-3 (1878). (2) Gilbert, F. L., Laxton, F. C., and Prideaux, E. B. R., J. Chcrn. SOC., 1927, 2295-308. RECEIVED July 18, 1935.
Determination of Small Amounts of Potassium by Means of Silver Cobaltinitrite REX J. ROBINSON AND GARTH L. PUTNAM, University of Washington, Seattle, Wash.
THE
4 determination of small quantities of potassium is of prime importance in the study of biological fluids, soil extracts, and fresh waters. Because of incomplete precipitation when there is less than 0.100 mg. of potassium per ml., the widely used method based on precipitation with sodium cobaltinitrite leaves much to be desired (7, 1 1 ) . Even when the potassium concentration is 0.150 mg. per nil. or more, satisfactory results are obtained only because of compensation of errors of rather large magnitude (7, 13). Taylor (14) has shown that the sensitivity of the method is increased with an alcoholic precipitation medium. Reported below are preliminary experiments with the sodium cobaltinitrite reagent establishing the lower limit of each of these procedures. From these data it is apparent that neither method possesses the sensitivity necessary for the direct determination af extremely small amounts of potassium, such as are found in most lake and river waters. Since concentration by evaporation in quartz or platinum is time-consuming and requires expensive equipment, a direct determination would be most desirable for water analysis. The silver cobaltinitrite method of Breh and Gaebler (2) appeared to offer the most promise for such a direct determination. Using a modification of this method, smaller amounts of potassium than reported elsewhere in the literature have been determined.
Colorimetric Determination of Nitrite Heretofore the most sensitive method for the determination of potassium has been based upon the precipitation of (Na,K)3Co(N02)6with subsequent determination of the cobalt or nitrite colorimetrically ( 7 ) . The extremely sensitive Greiss method has been extensively used for the colorimetric estimation of the nitrite content of this precipitate ( 5 ) . A solution containing 0.0005 t o 0.0050 mg. of PROC~EDIJRE. NOz ( I ) , which is equivalent t o approximately 0.0001 to 0.0010 mg. of potassium, respectively, was diluted t o 50 ml. in a Nessler tube, and n-as treated with a mixture of 2 ml. of sulfanilic acid and 1 ml. of 1-naphthylamine (1). Comparison was made with standards after the color had developed 20 minutes. To determine 0 005 to 0.100 mg. of NOz the solution was diluted to 100 ml., the color developed and the comparison with standards made in a Klett colorimeter. OBSE:RVATIONS ON THE COLORIMETRIC TECHNIC.From investigations of the colorimetric technic, the authors draw the following conclusions: First, to obtain strictly reproducible results the sulfanilic acid and 1-naphthylamine should first be mixed and then added t o the nitrite solution. When the Greiss reagents are added separately, the color is, within certain limits, more intense the longer the time interval between the addition of the sulfanilic acid and the 1-naphthylamine. This observation has been noted by others (7, Q), though most investigators fail to heed this precaution. The mixture of reagents was found to be stable for at least 2 weeks in the absence of light (9). Secondly,
confirming the work of Taylor (14) it was found that the use of a 10 per cent acetic acid medium prevents the formation of a reddish brown precipitate with large quantities of nitrite. Furthermore, when acetic acid and sodium acetate are used, they must be present in the same concentration in both standard and unknown (16). Thirdly, Beer's law is valid over a range of 0.0001 to 0.0010 mg. of NOz per ml. of solution, which is equivalent to 0.00002 to 0.00020 mg. of potassium per ml.
Determination of Potassium with Sodium Cobaltinitrite Reagent For the purpose of ascertaining the sensitivity of the sodium cobaltinitrite reagent, the authors employed a modification of the method (16) which Tischer had claimed to be more sensitive than previous methods. More recent work indicates that the Kramer-Tisdall (6) reagent may be somewhat more sensitive, though surely not less than 0.025 mg. of potassium per ml. can be determined with accuracy (12). In the present investigation the modifications of the Tischer method were: (1) the precipitate was separated by centrifuging; (2) the precipitate was dissolved in 0.1 N sodium hydroxide; and (3) the nitrite was determined with the Greiss reagents. The first two modifications were recommended by Lewis and Marmoy ( 7 ) ,while Herzner ( 5 ) showed that the Greiss reagents may be used satisfactorily for the determination of nitrite. The results with this modified procedure were corrected for a blank which ranged from 0.00002 to 0.0007 mg. of potassium. S o precipitation was observed when the potassium concentration was less than 0.040 mg. per ml., although a precipitate of (Na,K)&o(N02)~containing 0.001 mg. of potassium may be seen easily in the centrifuge tube. Moreover, seeding the reagent just before use did not increase the sensitivity of the reagent. It was found that a minimum of 0.120 mg. of potassium per ml. may be determined with accuracy when calculating the potassium from a potassium-nitrite factor based on the NaKzCo(NO& formula as Tischer did, whereas somewhat less than 0.080 mg. may be estimated quantitatively if potassium standards are carried through simultaneously with the unkhowns for the colorimetric comparison. Taylor (14) obtaind an increased sensitivity with the sodium cobaltinitrite reagent by the use of a precipitation medium containing about 30 volume-per cent ethyl alcohol. Recently Lohse (8) reported results for Taylor's procedure when the precipitation was made a t low temperatures. However, the smallest concentration of potassium reported by Lohse was 0.100 mg. per ml. and, other than this, the effect of cooling upon the sensitivity was not given. In Table I the sensitivity with the Taylor procedure is given for both room temperature and 4' to 6' C. The colorimetric com-
212
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 8, NO. 3
Silver cobaltinitrite reagent containing 1.0 per cent of silver nitrate was prepared by dissolving 25 grams of sodium cobaltinitrite in 150 ml. of sodium nitrite solution, containing 50 grams of sodium nitrite, and adding with stirring 5 ml. of silver nitrate TABLEI. POTASSIUM RECOVERY WITH TAYLOR'S PROCEDURE solution, containing 2.0 grams of silver nitrate. The solution was diluted to 200 ml., 2 ml. of glacial acetic acid were added, and Potassium at 4' to 6' C. Potassium at Room Temperature air was passed through the cold solution for 5 minutes. After Present Found Difference Present Found Difference standing for about 12 hour3 at 4" t o 6" C., the reagent was filMg./ml. Mg./ml. Mg,/ml. Mg./ml. Mg./ml. Mg./ml. tered through a No. 42 Whatman filter paper. 0.0020 0.0000 -0.0020 Fresh reagent was prepared at least once every 2 weeks and 0.0000 -0.0020 0.0000 -0.0020 was preserved at 4" to 6" C. when not in use. Just before use a 0.0030 0.0000 -0.0030 0.0030 0.0037 +0.0007 portion was centrifuged and the supernatant solution used for 0,0000 -0.0030 0.0019 -0.0011 precipitation of the potassium. 0.0000 -0.0030 0.0030 0.0000 A stock solution of potassium sulfate was prepared to contain 0.0060 0.0060 0.0000 0.0060 0.0056 -0.0004 1.000 mg. of potassium per ml. Standard solutions were pre0.0065 + O , 0005 0.0061 SO.0001 0.0060 0.0000 0.0060 0.0000 pared by proper dilution. 0.0120 0.0121 +0.0001 0.0100 0.0100 0.0000 Svnthetic hard water. Dotassium-free. had the followine com0.0120 0.0000 0.0091 -0.0009 poskion in parts per mhhon: Ca+, 195; Mg++, 53.3;- Na+, 0.0120 0.0000 0.0112 +0.0012 780; Fe+++, 35; SiOa--, 150; GI-, 1080; Sod--, 582. The 0.0300 0.0300 0.0000 0.0200 0.0201 +0.0001 solution was 0.17 N in respect to acetic acid. 0.0290 -0.0010 0.0200 0.0000 0.0300 0.0000 0.0190 -0.0010 PROCEDURE. One milliliter of a standard potassium solution was measured into a 15-ml. centrifuge tube and 1.00 ml. of the silver cobaltinitrite added. The precipitate and solution were On the basis of these results it can be stated that cooling allowed to stand for 3 hours and then centrifuged at 3000 r. p. m. enables the determination of smaller amounts of potassium. for 15 minutes. The supernatant liquid was carefully removed At 4" to 6" C., 0.0030 mg. of potassium was determined with through a siphon. The first washing was made with 5 ml. of water, the second with the same volume of 60 volume-per cent fair accuracy while a t room temperature no precipitate was acetone, and subsequent washings with the same volume of formed a t all. Furthermore, a comparison of the sensitivity 99.5 per cent acetone. After centrifuging 5 to 10 minutes, the of this method with that of the Tischer method confirms the wash liquid was removed by siphoning. The precipitate was conclusions of Taylor that an alcoholic precipitation medium finally dissolved in 1 ml. of 0.1 N sodium hydroxide. Solution was quickly completed by immersing the tube in a boiling water increases the sensitivity of the procedure. bath for 10 t o 15 minutes. The Greiss reagents were added and Formulas ranging from KNazCo(NOz)bto KzNaCo(NO& the colorimetric comparison was made against potassium standhave been reported as representing the oomposition of the ards that had been treated identically. When the amount of potassium was believed to be under 0.001 mg. the comparison precipitate, but the composition is now generally agreed to be was made in a colorimeter, using an aliquot containing 0,001 t o a mixture of these two forms whose ratio varies, depending 0.020 mg. of potassium. upon the conditions of precipitation, and is well represented by the formula (K,Na)3C~(NOz)a. However, with careful Following this procedure, known amounts of potassium regulation of conditions the composition is fairly reproducible in distilled water were precipitated both a t room temperature from one time to another. The formula vhich best repreand a t 0°C. to detprmine the sensitivity and the accuracy sented the product in the above determinations was calculated of the method a t the two temperatures. These results are to be Kl.&a~.65Co(T\TOz)6, assuming that all the potassium shown in Table 11. was precipitated. Using standard nitrite solutions for the colorimetric comparison, and a potassium-nitrite factor based RECOVERY WITH THE SILVER COBALTITABLE11. POTASSIUM on the latter formula, the weight of (K,Na)&o(NOz)6 precipiNITRITE REAGENT tated is readily calculated. When this was done the increased Potassium at 0' C. Potassium at Room Temperature recovery of potassium a t the lower temperature of precipitaPresent Found Difference Present Found Difference tion was evident. By this method not less than 0.030 mg. of potassium could be accurately estimated a t room temperature while as little as 0.006 mg. of potassium per ml. could be determined a t the lower temperature. 0.0020 0.0021 +0.0001 parison and estimation were made against standard potassium solutions treated simultaneously with the unknowns.
Precipitation with Silver Cobaltinitrite Silver cobaltinitrite reagent is the most sensitive of all precipitants of potassium (3, IO), yet, with the exception of Breh and Gaebler's (2) investigation in blood serum, there have been no attempts to apply this reagent to quantitative work. They stated that, with their procedure, it is possible to precipitate as little as 0.083 mg. of potassium from 5 ml. of blood serum filtrate and by using a calibration curve to obtain the results with an accuracy of 3 to 5 per cent. Although i t has been reported (3) that silver potassium cobaltinitrite has a pronounced tendency to become colloidal in the absence of alcohol and electrolytes, Breh and Gaebler used distilled water as the wash liquor. I n the following experiments acetone was the wash solution; alcohol was also tried but acetone was superior. An all-glass wash bottle was essential for use with either alcohol or acetone, as unsaturated compounds were often leached from the rubber connections with subsequent reduction of the silver in the precipitate. STANDARDS AND REAGENTS.I n all instances the reagents were prepared from purified c. P. chemicals. Alcohol and acetone were of such purity that neither gave a perceptible precipitate when treated with silver nitrate, allowed to stand for 3 hours at room temperature, and then centrifuged.
0.0050
0 0000 0,0000
0.0100
0.0500 0.1000
0.0000 0,0091 0.0113 0.0100 0.0486 0.0507 0.0500 0.1000
0.0993 0.1030
-0.0050 -0.0050 -0.0050
0.0050
-0.0009 +0.0013 0.0000 -0.0014 +0.0007 0.0000 0.0000 -0.0007 S0.0030
0.0100 0.0400
0 I0020 0.0019 0.0050 0.0050 0.0047 0.0100 0.0109 0.0092 0.0400
0.0404
0.0000 -0.0001 0.0000 0.0000 -0.0003 0.0000
+0.0009 -0,0008 0.0000
10.0004
With this reagent cooling assisted precipitation greatly and increased the sensitivity. When standard potassium solutions were carried through simultaneously with the unknowns a minimum of 0.010 mg. of potassium per ml. could be determined with precipitation a t room temperature, whereas 0.002 mg. per ml. could be determined a t 0" C. Moreover, precipitation was completed within 2 hours a t the lower temperature. The composition of the precipitate was found to be best represented by the formula K ~ . W ~ ~ I . ~ , & O ( N O ~ ) ! . Using standard nitrite solutions for the colorimetric comparison, and a potassium-nitrite factor based on the latter formula, the potassium content of the unknown was calculated. I n this manner a minimum of 0.050 mg. of potassium per ml. could
MAY 15, 1936
ANALYTICAL EDITION
be determined with precipitation a t room temperature and 0.002 a t 0 ” C. Although an alcoholic precipitation medium increases the sensitivity of the sodium cobaltinitrite reagent, neither alcohol nor acetone was found to exert a beneficial influence upon the sensitivity of the silver cobaltinitrite reagent.
.Determination of Potassium in Water Ammonium and chloride (3) are the only commonly occurring ions in water that interfere with the qualitative silver cobaltinitrite test. In view of the previous satisfactory results by this method, precipitation of the potassium without first concentrating appeared feasible when the potasssium occurred in quantities greater than 0.002 mg. potassium per m1.-that is, 2 p. p. m. Less than 0.002 mg. per ml. necessitated concentrating in an acid solution. Acetic acid was satisfactory for this purpose and concentrations as great as 0.08 N did not interfere materially with the precipitation. The effect of the excess silver ion upon the recovery of potassium was determined by using reagents containing 1 and 2 per cent of silver nitrate. For practical purposes the concentration of the excess silver ion in the precipitation medium was twice as great with the second silver nitrate solution as with the first because, in either case, only an extremely small amount of silver was removed by precipitation. When using the more concentrated solution, somewhat larger amounts of potassium were apparently recovered; this was believed to be due to a higher silver-potassium ratio in the precipitate. Prefera,bly but a slight excess of silver ion should be present from the precipitation of the chloride and, what is more important, the same concentration of excess silver should occur in both standard and unknown. In the analysis of naturally occurring waters, if the water sample is very soft, distilled water is satisfactory for the dilution of the potassium standards; while for accurate analysis of waters of greater hardness a synthetic water of the same hardness as the unknown should be used for this purpose. The previous work with the silver cobaltinitrite reagent indicated the accuracy which may be expected with very soft water samples. TABLE111.
POTASSIUM
RECOVERY FROM HARD WaTER
Present
-PotassiumFound
Mg./ml.
Mg./ml.
0.0020
0.0021 0.0020 0,0067 0.0060 0.0065 0.0061 0.0100 0.0094 0.0102 0.0097 0.0201 0.0200 0.0190 0.0400 0.0402 0.0392
0.0060
0.0100
0.0200 0.0400
Difference Mg./ntl. +0.0001 0.0000
+0.0007 0 0000 -0.0005 I
so.0001 0.0000 -0.0006 +0.0002 -0.0003 +0.0001 0,0000 -0.0010 0 * 0000 +0.0002 -0.0008
The applicability of the method to hard waters was demonstrated by the recovery of potassium from a synthetic water of extreme hardness. A potassium-free synthetic water was prepared which contained the maximum concentration of each substance ordinarily found in hard waters (4). The magnetiium and calcium for this solution were weighed as the carbonates and then dissolved in an excess of acetic acid; the excess acid prevented hydrolysis and precipitation of iron, etc. This solution was 0.17 N in respect to acetic acid, but after the addition of potassium and removal of chloride the acidity of the final potassium standards had been reduced to 0.05 N .
213
For the determination of potassium from the synthetic water medium, 30 ml. of standard potassium solution were added to 10 ml. of 8.3 per cent silver nitrate solution and then enough synthetic water was added to make a final volume of 100 ml. After thoroughly mixing, the precipitate was removed from a portion of the solution by centrifuging, Remaining in the supernatant solution was 0.55 per cent silver nitrate and 0.6 per cent acetic acid. The potassium content of this solution was determined with the silver cobaltinitrite reagent. The standards possessed the same ionic concentrations as the unknowns and were accorded the same treatment,. The results are shown in Table 111. The recovery of potassium from the hard water medium was remarkably good, considering the small quantity of potassium and the large quantity of other ions involved. Moreover, the analyses were made with greater rapidity and accuracy than could be expected with the normal evaporation procedure.
Summary When a 10 per cent acetic acid medium is used in the Greiss method for the determination of nitrite, Beer’s law is valid over a wide range. Since the intensity of the color is somewhat dependent upon the time interval between the addition of the sulfanilic acid and 1-naphthylamine, the accuracy of the method is improved by mixing the reagents before use. The Tischer method for potassium is not applicable t o the determination of less than 0.120 mg. per ml. of solution. Somewhat less than 0.080 mg. per ml. may be determined if comparison is made with potassium standards that have been given an identical treatment. By precipitating from a 30 volume-per cent alcoholic medium a t 4 O to 6 O C., the minimum quantity of potassium determinable may be decreased to about 0.003 mg. per ml. of solution. By employing a silver cobaltinitrite precipitating reagent, and running potassium standards simultaneously, the minimum quantity of potassium determinable is reduced to about 0.002 mg. Precipitation should be allowed to take place near 0’ C., and an acetone wash liquor used. This method was found to be applicable to the determination of potassium in fresh water.
Literature Cited (1) Am. Public Health Assoc., “Standard Methods for the Examination of Water and Sewage,” 7th ed., pp. 19-20, New York, 1933. (2) Breh, F., and Gaebler, 0. H . , J . B i d . Chem., 87, 81 (1930). (3) Burgess, L. L., and Kamm, O., J . Am. Chem. Soc., 34, 652 (1912). (4) Habermeyer, G. C., Illinois Dept. Registration and Education, Bull. 21 (1931). ( 5 ) Herzner, R. A., Biochem. Z . , 237, 129 (1931). (6) Kramer, B., and Tisdall, F. F., J . B i d . Chem., 46, 339 (1921). (7) Lewis, A. H . , and Marmoy, F. B., J . SOC.Chem. Ind., 52, 177T (1933). (8) Lohse, H . W., IND.ENQ.CHEM.,Anal. E d . , 7, 272 (1935). (9) Lunge, G., Z . angew. Chem., 23, 666 (1889). (10) Lute, O., Z . a d . Chem., 59, 145 (1920). (11) Piper, C. S., J . Xoc. Chem. Ind., 53, 392T (1934). (12) Rozanov, S. N., and Kazarinova, V. A., Z. anal. Chem., 96, 26 (1934). (13) Shedd, 0. M., J. IXD.ENG.CHEY.,1, 302 (1909). (14) Taylor, F. H . L., J.Biol. Chem., 87,27 (1930). (15) Tischer, J., Biochem. Z., 238, 148 (1931). (16) Uzel, R., Collection CzechosZov. Chem. Communications, 5, 139 (1933). RECEIVED February 11, 1936.
